“She’s looking at the cases and the linkages,” Det. Insp. Hillman, who’s in charge of the multijurisdictional investigation, said in an interview.

Dellen Millard, 28, and Mark Smich, 26, have been charged with first-degree murder in the May death of Mr. Bosma, a father from Ancaster, Ont. The pair made brief appearances in a Hamilton courtroom on Tuesday. Their next court date was set for Oct. 31, when they will appear by video.

Police have linked the Bosma investigation to two other mysterious cases tied to Mr. Millard: the disappearance of his friend Laura Babcock in July, 2012, and the death of his father Wayne Millard in November, 2012, which was originally deemed a suicide. The case has since been reopened. Det. Insp. Hillman wouldn’t elaborate on the connections.

This summer, the three cases were combined into what’s formally known as a multijurisdictional major-case investigation. Such a move occurs when “a reasonable likelihood exists that the same person(s) has committed the crimes,” according to the Ontario Major Case Management Manual, which was written as part of the government’s response to a 1996 report detailing systemic failures in the Paul Bernardo investigation. When that criterion is met, the serial predator crime investigations co-ordinator is notified and becomes part of the probe. Sgt. MacDonald was notified on July 30.

Det. Insp. Hillman stopped short of saying whether Mr. Millard or Mr. Smich are considered suspects in Ms. Babcock’s disappearance or Wayne Millard’s death. Neither has been charged in relation to the two cases.

“As the manager of an investigation, I have to keep a very open mind,” Det. Insp. Hillman said.

Toronto lawyer Tim Danson, who represented the families of two of Mr. Bernardo’s victims, said the “only reasonable inference” to draw from Sgt. MacDonald’s involvement is that police suspect a serial predator was involved. “If they’re bringing in this individual, who has this specific role, then how can it be anything else?” he said.

Mr. Millard’s lawyer, Deepak Paradkar, played down the co-ordinator’s involvement, saying the “reasonable likelihood” investigative standard is a “very, very low threshold.”

“I don’t see this implicating my client whatsoever as being a serial predator,” he said. “Serial, to me, it’s somebody who acts in a similar manner with similar crimes, similar victims. These are, to me, completely distinct cases.”

Sgt. MacDonald has been a Waterloo Regional Police officer for more than 20 years and became co-ordinator in 2011. Officials declined to make her available for an interview.

Don Forgan, a retired Hamilton detective who served as co-ordinator from 2008 to 2011, described Sgt. MacDonald as a “very well-respected” investigator. He said the co-ordinator’s involvement can range from deep to a matter of formality, but that the “major function” is communication.

“The importance of the role is making sure that investigators communicate with each other, from different jurisdictions,” he said.

While the Bosma, Babcock and Wayne Millard cases have been combined into a multijurisdictional investigation, Det. Insp. Hillman said they have not been integrated, meaning Hamilton and Toronto police are still investigating their own cases. Hamilton is in charge of the Bosma probe, while Toronto is investigating Ms. Babcock’s disappearance and Wayne Millard’s death.